Dr. Yakup Gönüllü Profile

Dr. Yakup Gönüllü

at Univ zu Köln

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Publications (3)

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Proceedings Article | 4 June 2014 Paper

Use of nanostructured oxides for selective gas-sensing

Y. Gönüllü, B. Saruhan

Proceedings Volume 9083, 90830Z (2014) https://doi.org/10.1117/12.2053000

KEYWORDS: NOx, Sensors, Resistance, Carbon monoxide, Chromium, Doping, Annealing, Gas sensors, Titanium, Titanium dioxide

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Titania nanotubular layers (TiO₂-NTs) are better known with their use in dye-synthesized solar cells. In the field of gas sensors, the best known application of TiO₂-NT layers is given with H₂ sensing. TiO₂–NT structures are commonly synthesized from pure titanium by anodic oxidation method. As literature indicates, gas sensing with titania can be modified by doping to obtain p- or n-type semiconductor behaviour. As undoped titania is widely used for CO and H₂- sensing, Al- and Cr-doped titania is reported to be effective for NO₂-sensing. Nanostructuring of TiO₂ yields faster and more stable response toward CO and NO₂ with almost no drift in sensor signal. Dopants are introduced by wet-chemistry method or by simultaneous anodization in an alloy. We introduced Cr³⁺ to TiO₂ NT-layers by soaking in nitrate salt and Al³⁺ by anodization of Ti6Al4V alloy. All doped TiO₂ NTs are tested for NO₂ sensing at temperatures up to 500°C in the presence of CO. The characteristics of doping are investigated by XRD, EDX-TEM methods. Cr-doping of nanotubular TiO₂ sensors displays p-type semiconductor behaviour and a considerably increased NO₂-selectivity, while Aldoping emphasizes the CO-selectivity. Effect of Cr-doping on sensing is further investigated by means of impedance spectroscopic measurements and equivalent circuit modeling in order to optimize the sensors for real-time measurements.

Proceedings Article | 28 May 2013 Paper

Pseudocapacitive and hierarchically ordered porous electrode materials supercapacitors

B. Saruhan, Y. Gönüllü, B. Arndt

Proceedings Volume 8728, 872810 (2013) https://doi.org/10.1117/12.2015275

KEYWORDS: Electrodes, Capacitance, Oxides, Wind energy, Crystals, Manganese, Capacitors, Sputter deposition, Magnesium, Titanium dioxide

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Commercially available double layer capacitors store energy in an electrostatic field. This forms in the form of a double layer by charged particles arranged on two electrodes consisting mostly of active carbon. Such double layer capacitors exhibit a low energy density, so that components with large capacity according to large electrode areas are required. Our research focuses on the development of new electrode materials to realize the production of electrical energy storage systems with high energy density and high power density. Metal oxide based electrodes increase the energy density and the capacitance by addition of pseudo capacitance to the static capacitance present by the double layer super-capacitor electrodes. The so-called hybrid asymmetric cell capacitors combine both types of energy storage in a single component. In this work, the production routes followed in our laboratories for synthesis of nano-porous and aligned metal oxide electrodes using the electrochemical and sputter deposition as well as anodization methods will be described. Our characterisation studies concentrate on electrodes having redox metal-oxides (e.g. MnO_x and WO_x) and hierarchically aligned nano-porous Li-doped TiO₂-NTs. The material specific and electrochemical properties achieved with these electrodes will be presented.

Proceedings Article | 3 May 2012 Paper

Electrochemical high-temperature gas sensors

B. Saruhan, M. Stranzenbach, A. Yüce, Y. Gönüllü

Proceedings Volume 8373, 83730Y (2012) https://doi.org/10.1117/12.918435

KEYWORDS: Sensors, NOx, Oxygen, Carbon monoxide, Gas sensors, Annealing, Magnetic sensors, Argon, Humidity, Titanium dioxide

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Combustion produced common air pollutant, NOx associates with greenhouse effects. Its high temperature detection is essential for protection of nature. Component-integration capable high-temperature sensors enable the control of combustion products. The requirements are quantitative detection of total NO_x and high selectivity at temperatures above 500°C. This study reports various approaches to detect NO and NO₂ selectively under lean and humid conditions at temperatures from 300°C to 800°C. All tested electrochemical sensors were fabricated in planar design to enable componentintegration. We suggest first an impedance-metric gas sensor for total NOx-detection consisting of NiO- or NiCr₂O₄-SE and PYSZ-electrolyte. The electrolyte-layer is about 200μm thickness and constructed of quasi-single crystalline columns. The sensing-electrode (SE) is magnetron sputtered thin-layers of NiO or NiCr₂O₄. Sensor sensitivity for detection of total NOx has been measured by applying impedance analysis. The cross-sensitivity to other emission gases such as CO, CO₂, CH₄ and oxygen (5 vol.%) has been determined under 0-1000ppm NO. Sensor maintains its high sensitivity at temperatures up to 550°C and 600°C, depending on the sensing-electrode. NiO-SE yields better selectivity to NO in the presence of oxygen and have shorter response times comparing to NiCr₂O₄-SE. For higher temperature NO₂-sensing capability, a resistive DC-sensor having Al-doped TiO₂-sensing layers has been employed. Sensor-sensitivity towards NO2 and cross-sensitivity to CO has been determined in the presence of H₂O at temperatures 600°C and 800°C. NO₂ concentrations varying from 25 to 100ppm and CO concentrations from 25 to 75ppm can be detected. By nano-tubular structuring of TiO₂, NO₂ sensitivity of the sensor was increased.

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